Superinfection of persistently infected patas monkeys with acute strains of simian hemorrhagic fever (SHF) virus has resulted in elimination of both the persistent and acute viruses, thus clearing the persistent infection. To date, confirmation of this result has been obtained in all 10 persistently infected animals, superinfected by this procedure. Our results suggest that immunoregulatory controls are involved in maintaining persistent SHF virus infections in the patas monkey and that these controls can be over-ridden by superinfection with acute strains of virus. Neutralizing antibody to persistent strains of SHF virus was not detected in sera of patas monkeys cleared of infection by superinfection. However, a persistent infection could not be re-established in the animals cleared of persistent infection. The mechanism of clearance of persistent infection did not appear to involve interferon. We are currently evaluating whether cellular immune mechanisms are involved in this clearance.